My car was like a sauna – until I tried this science-backed hack

Opening the car door during a heatwave feels like unleashing the unyielding violence of hell itself. Getting into that willingly is one thing, but shutting the door? A particularly sick form of masochism.

And yet it cannot be avoided. There are places to go, people to see and iced coffees to overpay for – and so we’ve got to find ways to cool our cars down.

If you are a forward planner, you might have already secured the spot in the shade on your street, keeping your car out of direct sunlight.

But for the rest of us who didn’t take mitigating steps, mathematician and broadcaster Hannah Fry has a recommendation. In a video viewed 10 million times since posting 11 months ago, she shares her own tip for cooling a car quickly as well as the science behind it.

Her tip is to open a window diagonal to your own car door and then, without shutting the door entirely, waft your car door rapidly. She explains that “the reason why it works is that when you open and close the door, especially if you do it quickly, as the door is moving outwards, it sweeps out all of the air that’s in the way”, aka the hot, muggy air. This creates an area of low pressure by the door which sets up bulk flow. “All of the horrible, hot air is drawn outwards, and the only way to refill it is with the cool, fresh air from the back”. And as if by magic (or physics), the car is cooled.

On one sweltering hot Monday in May, with temperatures reaching 34 degrees in London, I (reluctantly) put it to the test. From my window, I could see that the car had been baking in the sun ever since Saturday, when I’d done the groceries. Things got worse when I went outside. The heat in the air lay over me thickly, like a fluffy polyester blanket. In the two steps to the car, I was already sweating. Given that this May has been the hottest on record and a “Super El Niño” is predicted to emerge next year, bringing even hotter temperatures, this doesn’t bode well.

When I first got in the car and shut the door, I could immediately feel my skin dampening in a futile effort to cool me down. It was so hot and the air was thick and meaty with heat. Horrendous.

I promptly opened the rear diagonal window, hopped out of the car and started waving my car door around with all my might. On my first go (where I waggled my door back and forth about five times), the air didn’t feel significantly different so I went for another round. On my second go (when the door-waving was both faster and more frequent), it definitely felt more bearable but could still be better. On my third and final go (a very emphatic wave, quite fast, but only a few times), the air was marginally improved again.

But there were two problems. One was that the air outside my car was not dramatically cooler than the air inside. No matter how dramatically I sucked out the old stale air and pulled in the new fresh air, there wasn’t enough of a difference between them to make this feel like a real “a-ha” moment.

Secondly (and this could be a me thing), pushing and pulling a car door is not a significant workout by any means. But during a heatwave, when you’ve already been sauna’d by your car, it heats you quickly. And so even though the temperature had dropped a bit with my third round of fanning, my own body temperature had risen in the effort of doing so.

Dr Tom Crawford, mathematician at the University of Oxford and the University of Cambridge, agrees that this can reduce the efficacy of the door fan method.

“Things that may stop it working are if you open the door too slowly as then the air won’t be pushed away fast enough and so you fail to create the area of low pressure you need. Secondly, if you don’t open the back window enough to allow air to flow in quickly, once there’s space to do so.

And thirdly, if it’s the same temperature outside as in the car, it won’t make any difference even if you manage to replace the inside air with some from outside. “

So does it work? Yes – but with a catch. If I had done this first thing in the morning, when the air outside was still cool, it would have been a slam dunk. As it stands, when the UV rays were close to their peak, I was fighting a losing battle.

In any case, the door fan can only get you so far. The RAC also recommends pointing your air vents down (to facilitate the heat rising); using the “external” setting on the air conditioning to get fresher air; and opening your windows. Once the air that starts circulating is cold, then you can use the higher vents, start recirculating the air and close the windows.